There is an unusually high frequency of CTL clones that recognize allogeneic MHC molecules. While recognition of MHC determinants, per se, has been thought to underlay this phenomenon, recent structural, biochemical, and cellular studies have indicated that the peptide binding groove of the MHC molecule may normally be occupied by endogenously synthesized peptide. This suggests endogenous peptides may contribute to allorecognition. The applicant has recently obtained direct evidence that such peptide-dependent allorecognition is possible for CTL clones specific for H-2Kb. Further, peptides recognized in association with class I appear to be differentially expressed by cells of different origin. This proposal will analyze the generality of these findings and determine their applicability to define self and tumor antigens. CTL raised against Kb expressed on normal or tumor cell lines will be assessed for recognition of Kb expressed on normal cells, tumor cells, or cells of different species that have been transfected with the Kb gene. CTL clones which discriminate among such targets will be further analyzed to define the role of endogenous peptide in Kb recognition. Peptides prepared by cleavage of cytoplasmic proteins from cells recognized by such CTL will be used to sensitize Kb expressing cells which are not recognized. Where recognition appears to be specific for a tumor associated antigen, the capacity of such allogeneic CTL clones to eliminate tumor cells in vivo will be assessed. To learn more about the origin of the peptide antigens defined by allo-CTL, the hypothesis that they are derived from a select group of cellular proteins will be tested by biochemically purifying cytoplasmic proteins prior to cleavage into peptides. In all cases, proof that CTL clones recognize either class I plus a specific peptide or class I, per se, will be obtained using a cell free system for CTL triggering in which stimulatory antigen is provided in the form of purified immobilized class I molecules and specific peptide antigens.